Steam forming neutronic reactor and method of operating it



May l0, 1960 s. UNTERMYER STEAM FORMING NEUTRONIC REACTOR AND METHOD OF' OPERATING IT Filed June 28, 1955 14 Sheets-Sheet 1 Rw Vn mw. 4 /k .n P 01 m/ uw. X m w/ 9 M rn. m .whm

Affrney May 10, 1960 s. UNTERMYER 2,936,273

STEAM FORMING NEUTRONIC REACTOR AND METHOD OF OPERATING IT Filed Jupe 28, 1955 14 Sheets-Sheet 2 Wx l INVENTOR. Samuel Unfermyer BY Y Affofney Ma'ly 10, 1960 s. UNTERMYER 2,936,273

STEAM FORMING NEUTRONIC REACTOR AND METHOD OF OPERATING IT Filed June 28, 1955 14 Sheets-Sheet 3 INVENTOR. Samuel Untermyer BY Afrorne y May 10, 1960 s. UN'rl-:RMYx-:R 2,936,273

STEAM FORMING NEUTRONIC REACTOR AND METHOD OF OPERATING IT1 Filed June 28, 1955 14 Sheets-Sheet 4 May 10, 1960 s. UNTERMYER 2,936,273

STEAM FORMING NEUTRONIC REACTOR AND METHOD OF OPERATING IT 'Filed June 28, 1955 14 Sheets-Sheetl 5 Afforne y May 10, 1960 s. UNTERMYER 2,936,273

STEAM FORMING NEuTRoNIc REAcToR AND METHOD oF OPERATING `1T Filed June 28, 1955 14 sheets-sheet s FEE-12 n 4L-Normal, operating condition,

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Reacffvify change from steam voids in per cent FEE-if Enriched m y \zone l Mean core l radius Relative neutron flux I v i 0 l0 2O 30 40 50 60 70 8O Distance from center of reactor core in centimeters INVENTOR. Samuel Untermyer BY f /PM 4m Attorney May 10, 1960 s. UNTERMYER 2,936,273

STEAM F ORMING NEUTRONIC REACTOR AND METHOD OF' OPERATING IT Filed June 2, 1955 14 Sheets-Sheet '7 INVENTOR. Samuel unterm/yer A T TORNE' Y May l0, 1960' s. UNTERMYER 2,936,273

STEAM FORMING NEUTRONIC REACTOR AND METHOD OF' OPERATING IT Fired June 28, 1955 14 Sheets-Sheet 8 F/G f6 m .zza LIe196 f7 A /7 .vf Z-20 2512 1 i2 .ya l l n: 355 25a INVENTOR. Safrane( (Antartica/ef" A TTORNE Y May 10, 1960 s. UNTERMYER l 2,935,273

sTEAM EORMING NEuTEoNIc REAcToR AND METHOD oF OPERATING 1T yFiled June 28, 1955 14 Sheets-Sheet 9 INVEN1 OR. Sam nel un ter/@yer -1 ArroRNEY Mayy 10, 1960 s. UNTERMYER .STEAM FORMING NEUTRoNIc REAcToR AND METHOD oF OPERATING 1T Filed June 28, 1955 14 Sheets-Sheet 10 a a 4 2 a .6 i w e 6 z z y 2 2 A z IWI 2A 1| I-- Y., e Z. 2 F a W w Z, 8 0 3 Z a a 2 W fm a AZ I? w a 7 -..f Il Z 2 0 m w z w xm INVENTOR. .Samue (An tefvzjef' BY A TTORNE Y May 10, 1960 s. L lNTr-:RMYER 2,936,273

sTEAM FORMING NEUTRoNIc REAcToR AND METHOD oF OPERATING IT Filed June 28, 1955 14 Sheets-Sheet l1 INVENTOR. umuel du tcf'frg/er BY .A

- *kfz y ATTORNEY May 10, 1960 s. UNTERMYER 2,936,273

STEAM FORMING NEuTRoNTc REAcToR AND METHOD oF OPERATING 1T Filed June 28, 1955 14 Sheets-Sheet 12 INVENTOR. Samuel- Unfermyer BY ATTORNEY May 10, 1960 s. UNTERMYER .2,936,273

STEAM FORMING NEU'I'RQNIC REACTOR AND METl-IOD 0F OPERATING IT Filed June 28, 1955 14 Sheets-Sheet 1S INVENTOR. Samuel Unfermyer Attorney May 10, 1960 l s. UNTERMYER 2,936,273

STEAM FORMING NEUTRONIC REACTOR AND METHOD OF' OPERATING IT Filed June 28, 1955 14 Sheets-Sheet 14 F' q Z El Fi E E l] I I l an A I 44e 532 Is I mi# i 516' 5/6 50//5 506 9,32 70 kyZZ 6A- 52' INVENToR. Samuel Unfermyer fw Y Attorney Usted. l Sms Pacm- 2,936,273 STEAM FoRMiNG NEUTRoNrc REAcroR AND METHOD von OPERATING rr Samuel Untermyer, Scotia, N.Y.,Aassignor to the United States of America as represented by the United States Atomic Energy Commission Application June 28, 1955, Serial No. 518,427

13` Claims. (Cl. 20d-154.2)

rThe present invention relates to neutronic reactors and to methods -of operating neutronic reactors, and'more 'particularly to neutronic reactors and methods of operating reactors for the production of useful power. Prior to the present invention, every effort was made to prevent the formation of bubbles within theactive portion of a reactor. .This was done in spite of the fact that more 'l heat can be removed by water which is converted into steam than can be removedvby water flowing through the region of heat generation without converting the water to steam.

One of the reactors known to the art removes thermal energy. from the active portion of the reactor by flowing a -liquid coolant under pressure through the active portion i `coolant through that tube decreases, and the pressure 'drop across the nozzle decreases placing the entire manifold pressure on the tube in order to sweep bubbles out VIof the active portion of the reactor.

The Ireason that boiling of a liquid Within the active portion ofthe reactor was considered Yto be undesirable is that bubbling would aifect the reactivity of the reactor -and make steady control of the reactor difficult (Science ,property which can accomplish self-regulation. reactor@v may be constructed in which a reactivity increment changes the temperature of a reactor and the change in temperature in turn nulliiies the reactivity. increment.y This is another example of a self-regulating reactor. `,On the other hand, there are auto-catalytic reactors in which an increase in reactor power causes an increascinreac-Y tivity resulting in a further increase in reactor power. The effect of temperature on reactivity may again be used to achieve an auto-catalytic reactor, since reactors canbe constructed in which an increase in the temperature of,

the reactor further increases the reactivity of the reactor.

Throughout this disclosure, the denitionsusedare those conventionally used in the art as exemplified by The Elements of Nuclear Reactor Theory, by Glasstone and Edlung (1952), D. Van Nostranciy Company, Ine.'

' Some of the more frequentlyvused terms u sed herein` aref,

and Engineering of Nuclear Power, edited by Clark Goodman, Addison-Wesley Press, Inc., copyright 1947, page l' '303 -The formation of vapor bubbles within the active `portion of a reactor reduces the vdensityof the liquid y duced can be directly used to perform useful work. The

inventor has found that under certain conditions, the formation of vapor bubbles as a result of a neutron ux excursion from-the" injection of a reactivity increment into .the reactor will operate to nullify the reactivity increment 'within a suiciently short period of time to prevent un- :safe reactor operating conditions from developing. A reactor which behaves in this manner isa self-regu- Y lating reactor. vSelf-regulating reactors are those reactors which utilize an inherent physical property vto offset rchanges in reactivity which are injected into the reactor.

v` Bubble formation, or void formatiomis only' one physical defined as follows. f v The neutron multiplication factor, designated by-the; symbols "K, or Kw, defines the ratio of the number of neutrons in one generation to the number of neutrons.V

in the preceding generation of a reactor of infinite size with its control elements positioned for maximum` re-V activity. The neutron multiplication factor of av reactor is a function of the reactor materials and the disposition of these materials, and nota function ofthe mode of operation ofthe reactor.

The neutron reproduction ratio of the reactor, desig: nated Keff or "r, is the ratio of the number of neutrons in one generation to the number of neutronsin the preceding generation within a finite reactor structure, includ-v ing the eiects of the control mechanism for the reactor. The neutron reproduction vfactor is alwaysy smallery than the neutron multiplication ratio due to the fact that it takes into account the nte .size of the reactor, i and also is reduced by the effect of the control mechanism upon the reactor. lThe reactivity of a reactor, designated p, is defined by the expression .t

Kalf 1 Kaff the maximum reactivity ,of the reactor, pmm being the case in which Kaff is measured with the control mechanisms out of the reactor in the position of maximum reactivity. Y

The term steam forming liquid isa generic term lfor water and heavy water.k The term reactivity increment is used to denote a reactivity fraction, andlis generally used in this specification to describe the insertion of a re-L activity fraction to create reactor operating conditions under a condition of K excess, K excess being defined as Keff1 0. Y- i.

The active portion of a reactor is the region within the reactor shield including the fissionable material'and moderator material for the reactor. Generally, the active l portion has a core and a reflector surrounding thecore, and in breeder type reactorsit may include one or more blankets containing fertile materials.

The formation of bubbles, or an increase in the percentage of vapor within the active portion ofthe reactor, which is in effect an increase in the percentage of' voids in the active portion of the reactor, reduces Vthe' density of the liquid, and the resulting effect upon the re;-v activity of the reactor will depend upon the, particular construction of the reactor.' In general, one or more vof the following elfects will occur: 1 f.

(l) The decrease in the liquid density will reduce the moderating effect of they liquid, and hence theneut'ron flux within the reactor will have a higher energy.` As a result, the probability of a given neutron Withinthe active portion of the reactor escaping from .thejactve 2,936,273 M Patented May l l0, 1'9`60 portion is increased, and hence leakage from the reactor is increased.

(2) Since the increased temperature from the higher power and the decreased density of the liquid moderator results in a higher average neutron energy within the reactor, there will be a higher neutron resonance absorp tion in any U238 present in a thermal reactor.

(3) The decrease in the liquid density from the bubbles caused by the increase in reactor power decreases the probability of neutron absorption in the liquid itself.

(4) In reactors having fuel bodies, the increased temperature of the lissionable material bodies within the reactor as a result of the higher power level causes thermal expansion of fuel bodies. This factor tends to increase neutron leakage, and if U233 is present, this factor also tends to increase the resonance absorption of neutrons by reducing self-shielding in the fuel bodies.

In order to regulate a reactor by bubble formation, it is not only necessary that the bubble formation have a suicient effect on reactivity to overcome any reasonable reactivity increment injected into the reactor, but it is also necessary that the bubbles form in a suiciently short period of time to overcome a neutron flux excursion before damage has been done to the reactor structure.

The inventor has found that bubbles form suiciently rapidly in response to the insertion of a reactivity increment to compensate for this increase in reactivity before damage to the reactor structure can be caused, within limitations. The rate of rise of the neutron density within a reactor is a direct function of the magnitude of the excess reactivity. The inventor has found that a neutronic reactor operating at a temperature just below the boiling point of the reactor, and constructed according to the teachings of the present invention, will stabilize itself in the event a reactivity increment not to exceed is added to the reactivity of the reactor before damage to the reactor structure occurs.

It is an object of the present invention to provide a neutronic reactor which will directly produce steam within the active portion of the reactor; and to provide methods of operating reactors which result in the formation of steam in the active portion of the reactor.

It is a further object of the present invention to provide a neutronic reactor with a steam forming coolant capable of stable operation with the coolant, in a boiling state. In particular, it is an object of the present invention to provide such a reactor in which an increase in the reactor reactivity will be nulliied by an increase in the percentage of steam within the reactor active portion.

In addition, it is an object of the present invention to provide a reactor with a liquid moderator and a steam forming coolant which produces steam directly within the active portion of the reactor from the coolant, but avoids the formation of bubbles in the liquid moderator.

A further object of the present invention is to provide a reactor with a boiling steam forming coolant in which the coolant is circulated by convection, thus reducing the pumping power required to operate the reactor.

Further, it is an object of the present invention to provide a method of operating a reactor having a steam forming coolant in which steam is produced directly in the reactor core and changes in reactivity are compensated by changes in the power level of the reactor.

These and further objects of the present invention will be more fully understood from a further reading of the present specification, particularly when viewed in the light of the drawings, in which:

Figure 1 is an isometric view, partly cut away and in section, of a neutronic reactor constructed according to the teachings of the present invention;

Figure 2 is a vertical sectional view of the reactor illustrated in Figure 1;

Figure 3 is a somewhat schematic plan view of the core of the reactor illustrated in Figure 2;

Figure 4 is a vertical sectional view talren alongl the line 44 of Figures 6 and 7a, of one of the fuel elements for the reactor illustrated in Figures 1 through 3;

Figure 5 is a vertical sectional view taken along the line 5 5 of Figure 7b, of a second type of fuel element used in the reactor illustrated in Figures 1 through 3;

Figure 6 is a sectional view taken along the line 6-6 of Figure 4;

Figure 7a is a sectional view taken along the line 7a-7a of Figure 4;

Figure 7i) a sectional view taken along the line ib-7b of Figure 5;

Figure 8 is an elevational view from the line 8 8 of Figure 4;

Figure 9 is a sectional view of one of the control drive mechanisms illustrated in Figures l and 2;

Figure 10 is a sectional view of one of the control elements illustrated in Figures 2 and 3;

Figure 11 is a flow diagram showing the coolant circuit and the steam circuit for the reactor;

Figure 12 is a graph showing the relation of the ratio of the volume of steam in the reactor core to the total core volume relative to the reactivity of the reactor in percent of the neutron reproduction ratio, Km, at criticality;

Figure 13 is a graph illustrating the thermal neutron ilux pattern as it appears horizontally through the center of the reactor core, the relative neutron ux being plotted along the ordinate in percent, and the distance from the center of the reactor core Iin centimeters being plotted along the abscissa;

Figure 14 is a sectional View illustrating an electricity generating plant utilizing a boiling reactor;

Figure 15 is a plan view of the reactor shown in Figure 14;

Figure 16 is `a vertical sectional View of the reactor illustrated in Figure 15;

Figure 17 is a horizontal sectional view taken along line 17-17 of Figure 16;

Figure 18 is a vertical sectional view of the upper portion of a fuel element for the reactor illustrated generally in Figure 16;

Figure 19 is a vertical sectional view of the lower portion of the fuel element illustrated in Figure 18;

Figure 20 is a horizontal sectional view taken along the line 20-20 of Figure 18;

Figure 21 is a sectional view taken along the line 21-21 of Figure 18;

Figure 22 is a sectional view taken along the line 22-22 of Figure 19;

Figure 23 is a sectional view illustrating the cross-.section of the fuel ribbons used in the fuel element illustrated in Figures 18 and 19;

Figure 24 is a vertical sectional view illustrating a device for loading and unloading the fuel elements in the reactor best illustrated in Figure 16;

Figure 25 is a fragmentary view illustrating the portion of the control elements containing neutron-absorbing materials;

Figure 26 is a vertical sectional view of a third reactor constructed according to Ithe teachings of the present invention;

Figure 27 is a horizontal sectional view taken along the line 27-27 of Figure 26;

Figure 28 is a sectional view taken along the line 28-28 of Figure 26;

Figure 29 is a vertical sectional view taken along the line 29-29 of Figure 28;

Figure 30 is a fragmentary enlarged sectional view of a portion of the reactor core illustrated in Figure 26;

Figure 3l is an elevational view, partly in section, of one of the fuel elements used in the reactor illustrated in Figures 26, 27 and 30;

Figure 32 is a sectional view taken along the line 32-32 of Figure 31;

Figure 33 is a horizontal sectional view of the control rod of thereactor shown in Figure 26; and

.Figure 34 is a sectional view of a fragment of the fuel element illustrated in Figure 32.

A neutronic reactor which utilizes water (H2O) for both moderator and coolant may be constructed of virtually` any size and operated in a boiling condition if properly designed. However, in order to construct water cooled and moderated boiling reactors of small dimensions and large dimensions, it is necessary to use two different mechanisms to obtain inherent stability. In the case of small water cooled and moderated reactors, steam bubble formation results in an increase in neutron leakage from the reactor core` which is suilicient to achieve inherent stability. However, in large water moderated and cooled reactors, this effect is not adequate to achieve inherent stability. In both large and small water cooled and moderated reactors, inherent stability can be achieved if a suiicient quantity of U23B is present, since the forma- I tion of bubbles will increase the resonance absorption of neutrons in the U238 and achieve a negative powerreactivity relationship by this mechanism. The inventor has found that if the core of the reactor has a minimum cross sectional dimension greater .than three feet,` a water-cooled and ywater moderated reactor will not have. inherent stability by the leakage mechanism. The reactor therefore must includea quantity of U238 in order to'achieve inherent stability by the increased resonance v surrounding terrain. The reactor 70 is mounted withinv a cavity 76 in a concrete supporting structure 78, and a Alayer of thermal insulation 80 is disposed between the reactorV 70 and the walls of the ycavity 76.

f The reactor 70 is provided with a pressure vessel 82 which is cylindrical in shape and disposed vertically within the cavity 76. The pressure vessel 82 is provided with a neck 84 at its upper end with a diameter smaller than that of the pressure vessel 82, and the neck 84 forms.

an aperture 86 permitting access into the pressure vessel 82. A removable cover 88 is removably secured to the neck 84 of pressure vessel 82 about the periphery of the aperture 86 by spaced bolts 90 extending through the cover 88 and anchored into the neck 84 of the pressure vessel 82. The pressure vessel 82 includes a tank 91 and a stainless steel liner 92 on its inner Vsurface which prevents corrosiou ofthe steel tank 91. The inner diameter yof the stainless steel liner 92 is 6 feet 0 inch throughout the .cylindrical portion of the pressure vessel 82, and the innerv diameter of the neck portion 84 of the pressure vessel 82 is approximately 3 feet. The pressure vessel 82 rests upon a cylindrical tank support 93 which is secured to .the base of the tank 91 and rests upon the concrete supporting structure 78. The base of the tank 91 is in the form of a spherical segment 94 which supports the reactor core, designated 95, within-'the pressure vessel 82.

The neck 84 of the pressure vessel 82 is approximately 4 -feet long, and it takes approximately 1 foot to curve the pressure vessel 82 from its maximum diameter to the vdiameter of the neck 84. The height of the pressure vessel between the neck and the spherical segment base 94 is approximately 20- feet measured along the axis of f the pressure vessel 82. The pressure Vessel 82 is designed to withstand pressures upto 800 lbs. per square inch', and is intended to be operated at a pressure of 600 lbs. per square inch.

The cavity 76 which surrounds lthe pressure vessel 82A is also cylindrical in form with a diameter of approximately 8 feet. The layer of .thermal insulation 80 which is. attached to the exterior surface of the pressurervessel,v minimizes the escape of thermal energy from the interior l of the pressure vessel. The thermal insulation 480 in the particular construction here described is asbestbs and hasl a thickness of approximately 1 foot. Between theirk thermal insulation and the surfacesrof the cavity 76 is an air gap 96 which also thermally insulates the pressure vessel 82 from the supporting structure 78.

The supporting structure 78 is also provided with a i second cavity 97 directly beneath the first 'cavity 76, the two cavities being separated by a platform 98 of concrete which is about 3 feet thick. The pressure vessel 82 rests upon the platform 98, and an aperture 99 extends through the platform 98 directly beneath the pres'- sure vessel 82. The aperture 99 has two portions 100; and 101 disposed adjacent to each other, the upper portion 100 having a greater diameter than thelower por-V tion 101 of the aperture. A plug 102 in the form of a stepped cylinder has cylindrical sides conforming to those,

of the aperture 99 and is disposed within theapertureY 99. The plug 102 is provided with a plurality of chan-y nels 103 which extend therethrough parallel to the axisV of the pressure vessel S2 in order to accommodate movable control elements 104 for the reactor. A secondary steel tank 105 linestthe entirecavity 76 and also the walls of the aperture 99. The tank 105tis,

sealed to a plurality of control rod drive mechanisms 106 which extend into the second cavity 97V through the channels 103 in the plug 102. In this manner,ranyliquid leakage which occurs from the pressure vessel 82 will lbe retained within the secondary tank 105. Also, thek cavity 76 is surrounded by a plurality of coolant tubes` 107 embedded in the concrete supporting structure 78.

adjacent to the secondary tank 105, and a How of liquid coolant, namely water, in the particular construction, removes the heat which penetrates to the concrete s upporting structure 78 and prevents deteriora concrete therein.

The upper end of the cavity 76 (adjacent to the ground* level 74) is provided with a cylindrical region 108 with a larger diameter than the rest of the cavity forming a shoulder 109. A cylindrical concrete shield block 110 rests upon the shoulder 109 and'conforms in shape to the l Y surface of the cavity 76. A plug 111 is disposed within the neck 84 of the pressure vessel 82 and attached to the cover 88 thereof. The plug 111 and shield block 110 are constructed of concrete, and are also provided with coolant tubes 112 through which cooling water flows. A removable annular shielding slab 113 rests upon the ground surface 74 and extends over the neck 84 and annular shield block 110. It too is constructed of concrete.

The reactor core 95 rests upon an annular core supporting structure 114 which is provided with pedestals 1115 which are attached to the base 94 of the pressure vessel 82. The core 95 is formed by a plurality of removable fuel elements 116, each of the fuel` elements 116 being slidably disposed within a channel 117 whichV extends through the support structure 114 parallel to the Y axis of the pressure vessel 82. These removable fuel elements 116 contain thermal neutron issionable material for maintaining the neutron chain reaction and are of two types, 118 and 119. A neutron chain reactionv cannot be initiated and sustained with uranium having the isotopic content found in nature disposed within a.

body of water (H2O). It is for this reason that fuel elements 118 contain lissionable material consistingof uranium having the isotopic content found in nature,trV and fuel elements 119 contain ssionable material highly enriched in the U235 isotope, as Well as uranium withY the isotopic content found in nature.

As illustrated in Figures 4 and 5, both of the fut-.1`

elements 118 and 119 have-mounting stems 120v at one end which are removably positioned within channeled-17 in the support structure 114. The stems 1220 amplio- VA tion of they videdy with central channels 121 which communicate. with the exterior of the stems 125) through three apertures 122 at one end of the stems 120. The apertures 122 arerin the form of segments ot a circle formed by tins 123 which also serve to properly orient the fuel elements 116 within the support structure 114.

In the case of the elements 113, a pair of side plates 124 are attached to opposite sides of the stems 126 and extend to a rectangular discharge iitting 126, as illustrated in Figure 7b. The side plates 124 are each provided with four indentations 127, and the lndentations of confronting plates confront each other. Four plates 128 having thicknesses approximately equal to that of the indentations are disposed between the side plates 124 and anchored within confronting indentations 127.

The elements 11S have side plates 12d constructed of zirconium approximately 1/s inch thiol; and spaced from each other by a distance of 3% inches. The plates 128 are spaced from each other by a distance of approximately Sa inch, and the thickness of the plates 12S is approximately 3s inch. Each of the plates 12S is 48 inches long and 3% inches wide. The plates 123 have a body 1129 and a cladding 1311. The cladding 131i has a thickness oi .020 inch and consists of zirconium. The body 12% consists of 931/2% uranium having the isotopic content found in nature, 5% zirconium, and l/2% niobium.

The core supporting structure 111tis constructed of stainless steel, and the length of the channels 117 is approximately inches. The stems 121B of the fuel elements 11S and 11;@ have annular portions 131 for the rst 24 inches, and rectangular portions 132 adjacent to the side plates 126i. The diameter of the annular portions 131 is slightly smaller than the diameter of the channels 117 in the core supporting structure 114, namely approximately 31/2 inches. The discharge iitting 126 is hollow and rectangular in shape with approximately the same dimensions as the rectangular portion 132 of the stem 120. A plurality of orifices 133 are provided in each discharge litting 126, the rectangular end being open and forming a discharge port 134. The discharge fitting 126 is constructed of stainless steel and has a length of approximately 4 inches.

The fuel elements 119 include the same stems 12h and discharge fittings 126, but the regions between these structures are dilerent from that of the fuel elements 11S, as illustrated in Figures 6 and 7c. Side plates 13o constructed ot zirconium having a thickness of l; inch are secured between the stern 12d and the discharge fitting `126 of each fuel element 11? in a manner similar to the side plates 12st of the elements 113, but three aligned groups of live parallel spaced slots 137a, 137i) and 137C are disposed in the confronting surfaces of the side plates 136 and confront each other. `Five plates 13? containing uranium having the isotopic content found in nature are disposed between the iirst group of confronting slots 137a, this group being disposed adjacent to the stern 120 of the elements 119. immediately adjacent to the first group of aligned slots 137e, is the second group of slots 1.3715, and tive plates 139 containing uranium highly enriched the 11235 isotope. There are also live plates 1.33 containing uranium with the isotopic content found in nature disposed within the third group of confronting slots 137e in the side plates 1.35, this third group of slots 137C being disposed between the discharge itting 126 of the fuel elements 119 and the second group of slots 137i). The plates 13S are each 8 inches in length, and the plates containing uranium highly enriched in the H235 isotope are each 32 inches in length, the plates 13S being separated from the plates 139 by a distance of approximately 1/e inch.

The side plates 136 are constructed of zirconium and have a thickness of 1/s inch, width of 3% inches, and length of approximately 52 inches. The slots 137b in the second group are each approximately 1/16. inch indepth and have a width of. approximately 1/sfincln However, the slots 137a and 137k,` in the'other two groups have a depth of approximately 1/16 inch and width of approximately Ms inch. Each of theV plates 138 has a body 149 consisting of. 931/2% natural uranium, 5% zirconium, and Irl/2% niobium, and a cladding 141 2G mils thick consisting of zirconium which is disposed about the body 140 and seals it from the water moderator. The plates 133 have a thickness of approximately 1A inch, a length of 8 inches and a width of 3% inches. The plates have a body 142 consisting of a uraniumzirconium alloy containing 5% uranium and 95% zirconium, the uranium being enriched in the U235 isotope so that the U235 isotope constitutes 93.5% of the uranium in the alloy, the body 142 being 8G mils thick, 3% inches 'wi :and 32 inches long in each of the plates 139. ln addition, the plates 139 are provided with a cladding 143 of zirconium l0 mils thick. The plates 139 are spaced from each other by a distance of appoximately S; of an inch, and the plates 13S are spaced from each other by a distance of approximately 9/16 inch.

The channels 117 in the core supporting structure 114 are positioned to place the elements 116 in abutting relationship, so that the uranium containing plates 12S, 133 and 13h` are approximately equally spaced from each other and generally disposed on planes parallel to each other, as illustrated in Fig. 3. in the core of the reactor 7'ii, there are a total of 110 elements 11S and 38 elements 119 arranged to form an approximate foresnortened cylinder. The fuel elements 116 are disposed with their side plates 121i and 136 in parallel relationship in rows, the side plates 124 and 136 of adjacent rows abutting each other, and there being a total of 16 rows. As illustrated in Figure 3, the first row, designated 14351 contains two fuel elements 118, the second row 14311 contains tive fuel elements 113, the third row 143C contains eight elements 11S, the fourth row 14303 seven elements 113 and two elements 119 disposed centrally in the row, the next row 143e nine elements 11S and three elements 119, the three elements 11% being separated by, elements 118 and disposed between four elements 118 at one end and three elements 11S at the other end of the row. The next row 1li-3f contains eight elements 113 and four elements 119, the four elements 119 being separated by elements 118 and positioned between three elements 113 at one end of the row 143, and two elements 11S at the other end of the row 143i. The next row 143g contains seven elements 111i and live elements 119, the live elements 119 being positioned between elements 113, and two elements 118 being disposed at one end of the row 143g and one at the other end of the row. The following row 143k contains nine elements 11S and tive elements 119, the ve elements 119 being positioned between elements 11S, and two elements 118 being disposed at one end of the row 143k and three elements 118 being disposed at the other end of the row 14311. Thus far, the left half of the core $5 (as viewed in Figure 3) has been described, and the right half of the core 95 is constructed in similar manner, as illustrated in Figure 3. There are a total of thirty-eight elements 119 and 110 elements 118.

The reactor is also provided with twelve neutron absorbing control elements 1M. The controi elements 104 are disposed parallel with the axis of elongation of the pressure vessel 82. and are connected to the control rod drive mechanisms 1&6 by extension rods 144. The extension rods 144 extend through the channels 1113 in the plug 162, the extension rods 144 being surrounded by sleeves 1416 extending through the channels 1h13, the sleeves 14abeing sealed to the secondary tank 165. The control elements 1M are slidably journaled within housings 147, best shownV in Figure lO, which are secured within apertures 148 in the reactor core support structure 114. The housings 147 are rectangular in shape andV providedwithperforations-r149. The-housings 147 also ythird row 143C.

extend above the core 95 a distance of approximately 3 feet. The` housings 147 are constructed of zirconium and` have walls approximately 1/4 inch thick. The extension rods 144 are also constructed of4 zirconium and have a diameter of approximately 1/2 inch. The control elements 104 themselves are n the form of plates 150 of 1A inch thickness provided with a stainless steel cladding 151. The control elements 104 measure approximately 31/2 inches wide, 1/2 inch thick,and 4 feet long, the thickness of the stainless steel cladding 151 being 20 mils. The control elements 104 extend approximately l foot above the housings 147 when in the position of maximum reactivity, as illustrated in Figure 2.

One of the control rod drive mechanisms 106 is shown in detail n Fig. 9. The rod extension 144 extends through a housing 152 of the drive mechanism 106 and an electric Amotor 153 is mounted to the housing and has a rotor threadedly engaged with the extension rod 144. A

spring 154 is also disposed within the housing 152 and has one end abutting the end 155 of the housing 152 and the other end attached to the extension rod 144. The extension rod 144 is not rotatable within the housing 152 or sleeve 146, so that operation of the motor 153 translates the extension shaft within the sleeve 146 and housing` 152. In this manner, the spring 154 is compressed as the control yelement 104 is removed from the core 95 of the reactor. When the control element is to be reinserted into the core 95 of the reactor, the spring 154 supplies an addi-l tional acceleration, in the particular construction, an ac- A celeration equal to 2g.

As shown in Fig. 3, again viewing the gure from the left, the first row143a of fuel elements 116 contains no control elements 104, nor does the second row 143b, or The fourth row 143d, however, contains two 'control elements 104, these elements being spaced from the ends of the row 143d by three fuel elements 116. rThe next adjacent row 143e contains no control elements 104, nor does the next adjacent row 1433. However, the following row 143g contains four control elements 104, the outer two being disposed with three fuel elements 116 between them and the ends of the row 143g, and each of the other two control elements 104 being spaced by fuel element 118 and fuel element 119. The next row 143k again contains no control elements. This covers the left `half of the core 95, and the right half s of similar construction, as illustrated in Figure 3.

Figure l1 schematically illustrates the cooling water andl steam systems for the reactor. The water level, designated 156, within the pressure vessel 82 is disposed about halfway between the spherical base segment'94 of thev pressure vessel 82 and the neck 84 thereof. In this manner, a steam dome 157 is formed between the water level 156 and the neck 84 of the pressure vessel 82. The steam dome 157 functions to collect steam, form a cushion against pressure surges, and to delay the passage of steam kfrom the reactor core for a period of time, thus permitting aportion of the water carried by the steam to return to` the body of water within the pressure vessel 82. The pressure vessel 82 is provided with an aperture 158 Y slightly below the neck 84 thereof, and a steam separator Y and accumulator 159v is connected to the pressure vessel through this aperture 158. The steam separator and accumulator 159 further reduce the moisture content of the steam produced within the reactor and return the excess water `through a pipe-160 to the body of water within the reactor pressure vessel 82. The steam is conducted from the steam separator and accumulator 159 through a flow meter 161 and a turbine throttle valve 162 to a steam turP bine 163; The turbine 163ismechancally connected to a generator 164, and the steam exhausted from the turbine 163 is conducted to a condenser 166. The water condensate inthe condenser 166 is then pumped back into the pressure vessel 82 through a pair of parallelly connected pumps 167 and 168 which are provided with inlet valves 169 and outlet valves 170. Thewater condensate to the axis of elongation of the pressure vessel 82 and y adjacent to the inner surface of the pressure vessel 82.

The condenser 166 is cooled by a flow of water in a ciosed circulation loop. The water coolant is conducted through a pump 174 to a cooling tower 176 which reduces the temperature of the water to a suitable value, and. thence the Water is returned to the condenser'166.

The excess steam from the condenser 166 is pumped by a pump 177 to one of two holdup tanks 178' and 179. The holdup tanks 178 and 179 are connected to the pump 177 through inlet valves 180. The holdup tanks 178Aand 179 are also provided with outlet Valves 181 which connect the holdup tanks 178 and 179 to an air lter V182 and thence to a stack 183.

The steam Ydome 157 is provided with two pressure relief valves 184 and 186. The iirst pressure relief valve 184 exhausts directly to the atmosphere and is connectedV into the steam dome 157. It is adjusted to trigger at a pressure of 800 pounds per square inch. The second pressure relief valve 186 is also connected into the steam dome 157 but exhausts into the condenser 166. This pressure relief valve 186 is adjusted to trigger at a pressure of 700 pounds per square inch. In this manner, steam released by the pressure relief valve 186 will traverse the condenser 166, and hence one of the holdup tanks 178 or 179. In this manner, any radioactivity present in the steam will be permitted to partially decay in the holdup tank before being exhausted to the atmosphere.

A bypass control valve 187 is also connected between the inlet end of the turbine throttle valve 162 and the condenser 166. The dashed line in Figure 11 indicates that the turbine throttle valve 162 and bypass control valve 187 are interconnected, however, the bypass control valve 187 may be actuated independently from theY turbine throttle valve 162. When independently operated,

of the steam generated by the reactor to be directly connected to the condenser 166, thus bypassingtherturbine 163. In this manner, it is possible to operate the reactor without the turbine. However, in usual' opera-y tion the bypass control valve 187 is linked to the turbine throttle valve 162. The turbine throttlevalve 162 is itself linked with an overspeed governor 188 which is mechanically connected to the turbine 163, and when the overspeed governor 188 responds to an overspeed of the" turbine 166, the turbine throttle valve 162 is actuated to reduce the flow of steam into the turbine 163. This action generally occurs when the load upon the turbine.' 163 is greated reduced. Since a reduction in the iiow of. steam from the reactor wouldV build up pressure within the pressure vessel 82 ofthe reactor, thus reducing the voids formed by the steam bubbles in the reactor core An extraction type turbine is desirable in order to maintain the moisture content in the last exhaust stage'within acceptable limits (12 to 14%). VThis is accomplished by a steam separator 189 which is connectedbetween the pounds per squarein'ch,-V and an exhaust steamA pressure of 2%/2V inches of mercury.

11 stages of the turbine 162, the water removed from thesteam being returned to the condenser 166.

The control of steam start up is accomplished in the conventional manner, i.e., with the turbine throttle valve 162. Also, load regulation of the generator 164 is automatically accomplished by the overspeed governor 188 actuating the throttle valve 162 and bypass control valve 187, as described above.

In order to remove corrosion products from the water in the reactor, two separate filtering systems are incorporated into the reactor heat exchange system. Corrosion products will decrease the heat transfer from the fuel elements, and are therefore undesirable. One ion exchanger 190 is connected between the outlet valves 170 of the feed pumps 167 and 163 and the inlet valves 169 of the feed pumps 167 and 16S through an inlet valve 191, an inlet lter 192, an outlet filter 193 and an outlet valve 194'. This purification system bleeds off a portion of the condensateV from the condenser 166 and purities it before it is returned to the reactor pressure vessel 82. The second purification system employs an ion exchanger 196 connected to the pressure vessel 32 through a valve 197, a heat exchanger 198, and a filter 199. The ion exchanger 196 is also connected to the outlet end of the condenser 166 through a filter 200 and a valve 201. rhe heat exchanger 198 is provided with a iiow of coolant water to reduce the temperature of the water passing through the ion exchanger 196 to a suitable value for efiicient ion exchanger operation.

A make-up tank 202 is also connected to the condenser 166 through a valve 203. The make-up tank 202I is used to store filtered and purified water for the reactor and to introduce additional quantities of water into the cooling system. An external water level indicator 204 is connected to the pressure vessel 82 at a point below the water lever 156 and at a point above the water level 156 in order to determine the level of the water within the reactor pressure vessel 82.

It is desirable from a control viewpoint that the temperature of the water within the pressure vessel 82 be 212 F. or higher before any of the control rods 104 are removed from the core 95 of the reactor. It is therefore desirable that the water within the reactor pressure vessel 82 be brought to a temperature of 212 F. by nonnuclear means. For this reason, an external heater 206 is disposed adjacent to the pressure vessel 82 and connected to the pressure vessel 82 at two points below the water level 154. In the particular construction, the external heater 206 employs a source 207 of electric power to heat the water within the pressure vessel before start up, although it will be understood that other types of power could also be employed.

Table I summarizes some of the principal design characteristics of the reactor disclosed above:

TABLE I 6 ft. diameter, 19 It. high.

Pressure vessel 600 lbs. per sq. in.

Pressure in pressure vessel Moderator Volume ratio of H2O/D Core diameter Core height Maximum fuel eleme temperature. Maximum fuel element core temperature. Reactor power Generating capecity Efficiency Core volume Power density-average surface total. of core, 18 kwJliter of Power density-maximum 65 percent.

6.5 percent.

150,000 B.t.u.,/(hr.) (sq. t.). the 140:1.

to steam Internal circulation ratio, ratio ofjvater mass ow mass ow in the reactor.

flow to the turbine 163 during 112 TABLE IJ-Continued Fuel elements 118 93.5% natural uranium+ 5% Zr-l-l%% Nb.

% in. plates, 20

Approximately 4 it. high,

mil zirconium clad, in. water passage.

Fuel elements 119 Natural uranium ends 8 in. long. 30 assemblies in central zone of re- Enriched zone 32 in. long; Zr-U235 actor. alloy plates; zirconium clad, 10

mils; body, mils; 5% Um, Zr; water passage, in.

Critical mass, operating plus 2 10 kgs. U215 and 6 tons natural months buruup of enriched iuel. uranium. Operating cycle:

Enriched fuel 40% burnup, 2 years. T Naturalfuel 10,000 MWD/ton in l0 yrs.

of l-'il E michel .luium power Natural uranium power. Safety and control 12 boron-steel control rods in core; buruup, 'oy adding U235 fuel; transient, by insertion of poison rod. Effect of adding one enriched'uel 0.4% k average or 1.0% maximum.

Prompt neutron lifetime Radial flux, i.e., the ratio of the minimum thermil neatrou Iiux to the average thermal neutron ilus in a horizontal plano.

Axial nur, i.e., the ratio of the maximum thermal neutron flux to the average thermal neutron iiux along the central axis of the reactor. Neutron flux (average) l0u n /cm/sec.

The fuel elements 116 must be removed from the reactor from time to time. The reactor is designed to require replacement of the fuel elements 118, which contain uranium with the natural isotopic content, after 10,000 megawatt days per ton of uranium, or in other words, approximately every ten years. The fuel elements 119, which contain enriched uranium sections, will reqiure replacement ou an average of every two years. Also, some enriched uranium elements 119 will be required to replace fuel elements 118 after prolonged periods of operation in order to maintain the desired reactivity as a result of losses in reactivity which occur through long term operation.

Fuel elements 118 and 119 are removed through the neck 84 of the pressure vessel 82. Before the fuel elements 116 are to be removed, the reactor is shut down, and the entire pressure vessel I82 flooded with water, including the steam dome dome 157 is thus used as a shield when the removable slab 113, cover 88, and plug 111 are'removed. The fuel elements 116 are then individually lifted through the neck 84 of the pressure vessel 82 and placed within a shielded container, or coffin, as is conventional, for removal. A time lapse of approximately 3 hours is required between reactor shutdown and unloading operations With the particular construction to permit the radioactivity of the fuel elements l116 to decay sufficienly for removal from the pressure vessel 82. New fuel elements 118 and 119 arelowered into the reactor through the neck 84. and placed in the supporting structure 114.

The heat generated within the fuel elements 118 and 119 is removed by the natural circulation caused by the boiling of the coolant within the fuel elements 118 and 119. The coolant water enters the reactor at a point above the reactorcore 95 through the distribution ring 17 3. This water then flows downwardly, principally along the walls of the pressure vessel 82, to enter the fuel elements 116 at their lower extremity. The loss in density of the water in the fuel elements 116 as the water is heated and transformed to steam, causes the water to flow upwardly through the-fuel elements 116. At full power, for each-pound of steam formed approximately pounds of water are circulated as a result of convectiounths manner.. l

157. The Water in the steam. 

1. THE METHOD OF SAFELY OPERATING A NEUTRONIC REACTOR HAVING A STEAM FORMING COOLANT IN THE REACTOR CORE, WHICH COMPRISES PROVIDING A REACTOR HAVING A NEGATIVE REACTIVITY TO CORE VOID COEFFICIENT BETWEEN 0 AND 0.5% K PER PERCENT VOID AT THE REACTOR TEMPERATURE AT WHICH THE COOLANT COMMENCES BOILING, INITIATING A NEUTRONIC CHAIN REACTION WITHIN THE REACTOR BY INJECTING INTO THE REACTOR BETWEEN 0 AND 10% REACTIVITY GREATER THAN THE REACTIVITY REQUIRED TO PROVIDE A NEUTRON REPRODUCTION RATIO OF UNITY AT A TEMPERATURE JUST BELOW THE BOILING TEMPERATURE OF THE COOLANT, WHEREBY STEAM IS GENERATED IN THE CORE AND THE REACTIVITY INCREMENT IS NULLIFIED BY STEAM FORMATION IN THE CORE, AND INTRODUCING LIQUID COOLANT INTO THE CORE TO COMPENSATE FOR THE STEAM SO GENERATED. 